1. Technical Field
The present invention pertains to communication systems. In particular, the present invention pertains to secure initiation of communications within radio communication systems. The present invention may be applied to radio communication systems including individual communication units with a tuner capable of tuning to a wide set of frequencies. In exemplary embodiments, the present invention is applied to Spread Spectrum Systems (e.g., Direct Sequence, Frequency Hop and Hybrid) with individual communication units including a receiver capable of tuning to different frequencies, and the systems employing waveforms in the form of coded spread spectrum signals, where the signals are expanded in bandwidth via a spreading code.
2. Discussion of Related Art
Radio or wireless communication system operation typically includes an initial synchronization between transmitting and receiving units. A signal may be constantly transmitted into the environment to synchronize receiving units that are enabled at random times. This technique is employed by standard cellular communication systems, where a fixed base station constantly transmits a synchronization signal. In highly mobile systems without base stations, the synchronization is typically achieved in other fashions by the individual units. Further, critical systems (e.g., military or emergency management systems) require a high degree of reliability and resistance to intentional and/or unintentional spurious signals.
A manner of enhancing security and reliability within radio communication systems includes employment of encoded signals, such as spread spectrum signals. However, the processing required by the individual communication units to employ encoded signals significantly increases power consumption by those units. By way of example, spread spectrum communication systems are described to illustrate the significant processing performed by the individual communication units to employ encoded (e.g., spread spectrum) signals. In particular, the operation of a Direct Sequence Spread Spectrum (DSSS) communication unit includes spreading of a baseband signal (e.g., bandwidth expansion) by use of pseudonoise (PN) codes. The frequency rate of the codes greatly exceeds that of the baseband signal, where each transition or code symbol is commonly referred to as a “chip”. The codes or chips are basically modulated onto the baseband signal containing data and the resulting signal is mixed with an RF carrier signal and transmitted for reception by the appropriate receiving radio or communication units.
A cross correlation of an incoming signal with a suitable code replica by a digital matched filter (DMF) within the receiving communication unit produces an energy peak at the exact match, thereby rejecting or filtering other pseudonoise codes and background noise and interference. The power ratio between the incoming signal and the digital matched filter output signal is commonly referred to as processing gain (PG) and is defined by the filter length (e.g., quantity of filter stages) or quantity of chips per baseband signal. The power gain may be expressed in terms of decibels (dB) as:PGdB=10*log10 (quantity of chips/1 symbol) orPGdB=10*log10 (Chipping or Code Rate/Symbol Rate).For example, when each data symbol within the baseband signal is spread by one-hundred twenty-eight chips, the processing gain is:PGdB=10*log10 (128/1)=21 dB.Further, the above equations indicate a 32 MHz chip or code rate when the power gain is 21 dB and the symbol rate is 250 kilo symbols per second (ksps) with each data symbol spread by one-hundred twenty-eight chips.
Pseudonoise orthogonal or quasi-orthogonal codes may be selected to correlate exactly with a peak power output of the digital matched filter when the incoming signal code and replica codes match, and to produce a filter output of zero when these codes do not match (e.g., even if the codes are offset or shifted by one position). Operation of the digital matched filter is illustrated, by way of example only, in FIGS. 1–2. Specifically, an incoming signal including bi-phase modulated chips or codes is correlated with a stored replica code for a given time interval. Each chip within the incoming signal is multiplied by the corresponding chip in the replica code, where the individual products are summed to produce a correlation result. The filter operation may be expressed as:
      ∑          k      =      1        N    ⁢          ⁢            c      k        ⁢          r      k      where ck represents the incoming chip sequence, rk is the stored replica code and N represents the quantity of chips in the code.
Referring to FIG. 2, the filter is employed, by way of example only, with respect to short Barker codes. Specifically, an incoming signal with a code of 1, 1, 1, −1 is matched exactly to the stored replica code. The correlation value of four (e.g., the sum of the products of the corresponding chips, or (1×1)+(1×1)+(1×1)+(−1×−1)=4) indicates a peak power output of the filter. However, when an incoming signal with a code of 1, 1, −1, 1 does not match with a stored replica code of 1, 1, 1, −1, the filter provides a correlation value of zero (e.g., the sum of the products of the corresponding chips, or (1×1)+(1×1)+(−1×1)+(1×−1)=0) indicating the absence of a match.
Signal to Noise Ratio (SNR) is commonly defined as the ratio of the average signal power to the average noise power and is preferably measured in decibels. The average signal power is determined over the signal bandwidth, where the bandwidth for a spread spectrum signal is greater than that for a narrowband or baseband signal. For example, a baseband signal with 25 KHz of bandwidth and expanded at a ratio of one-hundred produces a spread signal with 25 KHz*100=2.5 MHz. The baseband signal includes, for a transmitted power of one Watt, a signal power of 1/25 KHz =0.040 Watts/Hz or 40 milliWatts/Hz, while the spread signal includes 1/2.5 MHz=400 NanoWatts/Hz. Thus, the spectral density of the narrowband or baseband signal contains more power in a narrow band, while the spread signal contains the same power for a sufficiently greater bandwidth with a lower peak power. This enables the spread signal to reside within or be camouflaged by the environment noise.
The pseudonoise codes provide a manner for a receiving unit to basically identify and differentiate between messages directed to that unit and other units, and further ensures that other transmissions within the surrounding environment are not erroneously considered as messages addressed to the receiving unit. Moreover, the pseudonoise codes enable secure and reliable communications in the presence of background and/or multi-user interference, while the bandwidth expansion of the baseband signal distributes the signal power over a greater bandwidth, thereby reducing the power spectral density of that signal in maximum amplitude and reducing visibility of the signal in the environment.
The security provided by the pseudonoise codes is based on the requirement that the receivers have knowledge of the specific code being transmitted in order to acquire and demodulate the signal. Accordingly, long or lengthy pseudonoise codes are utilized in some commercial systems, where subsections of the code are used to spread symbols with subsequent symbols being spread by different subsections. This is typically employed by military systems having security as a key requirement.
Accordingly, the spread spectrum receiving units perform significant processing to decode a transmitted signal, thereby substantially increasing unit power consumption. Further, the electronic circuitry of the receiving units include digital matched filters (DMFs) that match the transmitted codes to stored code replicas as described above. Thus, the receiving units must sample the environment and perform the filtering at the high chipping or code rate. For example, a baseband signal, b, that is expanded by N chips requires a digital matched filter of a receiving unit to perform numerous operations, Nops (e.g., including N multiplications and N−1 subsequent additions), to compare or match the stored code replica to the received signal as described above. Since the digital matched filter operates at the sampling rate which, to satisfy the Nyquist principle, must be at least twice the chip rate (i.e., two samples per chip), the following quantity of operations are typically performed:Nops=2*N (multiplications)+2*[N−1] (additions)These operations continue until the acquisition symbols (e.g., symbols within the transmitted codes) are exhausted. For example, the total quantity of operations, Tops, for S acquisition symbols may be expressed as:Tops=S*NopsThe filter continues to operate at this rate even during intervals of silence or low transmission rates in order to detect a transmission directed to that receiving unit. This especially occurs in systems employing Code Division Multiple Access (CDMA), where each communication unit or user is assigned a different code for communication.
With respect to Frequency Hop Spread Spectrum systems, the baseband signal is shifted in frequency based on a pseudonoise code to spread the signal over a wider bandwidth. The sequence in which the frequencies are transmitted is derived from the code, while the shifting or hopping rate is a function of the data or information rate. In order to acquire the signal, a receiving unit is typically required to shift or hop frequencies at a compatible rate in accordance with the spreading code. This similarly increases power consumption by the receiving unit. Hybrid Spread Spectrum systems typically employ a combination of direct sequence and frequency hopping techniques and thereby experience a compounded problem.
Since operation of a receiving unit to acquire an encoded transmitted signal significantly increases unit power consumption, the secure communication techniques described above are not economical or feasible for battery operated small devices, especially when there are long intervals of standby or silence.